Anion Gap & Metabolic Acidosis

The anion gap (AG) represents the amount of unmeasured anions in the plasma.

AG =([Na]+[K]) – ([HCO3]+[Cl])

The main contributor to the AG is albumin (decreasing albumin by 1g/l reduces the AG by 0.25) so hypoalbuminaemia can falsely reduce the AG.

Corrected AG = AG + (0.25*(40-[albumin]))

(However, this relies on getting LFT’s back about 1 hour)

What’s Normal?

The normal anion gap should be 6-16mmol/l

Why bother?

Knowing the anion gap can help you limit the differentials, of metabolic acidosis. (when you don’t know what is going on)

Metabolic Acidosis – Normal Anion Gap

  • RTA (Renal Tubular Acidosis)
  • Addison’s/Acetazolamide
  • GI (Diarrhoea, pancreatic and ureteric fistulae)
  • Excess Chlorine
  • Diuretic (spironolactone)

Metabolic Acidosis – Increased Anion Gap

You may have heard of CAT MUDPILES but in Yorkshire we have MILD TEACUPS

  • Methanol
  • Iron/Isoniazid/Inborn errors of metabolism
  • Lactate
  • DKA (and any other ketosis)


  • Theophylline/Toluene
  • Ethanol/Ethylene Glycol
  • Aminoglycosides (Gentamicin etc.)
  • Carbon Monoxide/Cyanide
  • Ureamia
  • Paracetamol/Paraldahyde
  • Salicylates